Molding is a process by virtue of which a molded article can be formed from molding material by using a molding system. Various molded articles can be formed by using the molding process, such as an injection molding process. One example of a molded article that can be formed, for example, from polyethylene terephthalate (PET) material is a preform that is capable of being subsequently blown into a beverage container, such as, a bottle and the like.
A typical molding system includes an injection unit, a clamp assembly and a mold assembly. The injection unit can be of a reciprocating screw type or of a two-stage type. The clamp assembly includes inter alia a frame, a movable platen, a fixed platen and an actuator for moving the movable platen and to apply tonnage to the mold assembly arranged between the platens. The mold assembly includes inter alia a cold half and a hot half. The hot half is usually associated with one or more cavities (and, hence, also sometimes referred to by those of skill in the art as a “cavity half”), while the cold half is usually associated with one or more cores (and, hence, also sometimes referred to by those of skill in the art as a “core half”). The one or more cavities together with one or more cores define, in use, one or more molding cavities. The hot half can also be associated with a melt distribution system (also referred to sometimes by those of skill in the art as a “hot runner”) for melt distribution. The mold assembly can be associated with a number of additional components, such as neck rings, neck ring slides, ejector structures, wear pads, etc.
As an illustration, injection molding of PET material involves heating the PET material (ex. PET pellets, PEN powder, PLA, etc.) to a homogeneous molten state and injecting, under pressure, the so-melted PET material into the one or more molding cavities defined, at least in part, by the aforementioned one or more cavities and one or more cores mounted respectively on a cavity plate and a core plate of the mold assembly. The cavity plate and the core plate are urged together and are held together by clamp force, the clamp force being sufficient enough to keep the cavity and the core pieces together against the pressure of the injected PET material. The molding cavity has a shape that substantially corresponds to a final cold-state shape of the molded article to be molded. The so-injected PET material is then cooled to a temperature sufficient to enable ejection of the so-formed molded article from the mold. When cooled, the molded article shrinks inside of the molding cavity and, as such, when the cavity and core plates are urged apart, the molded article tends to remain associated with the core. Accordingly, by urging the core plate away from the cavity plate, the molded article can be demolded, i.e. ejected from the core piece. Ejection structures are known to assist in removing the molded articles from the core halves. Examples of the ejection structures include stripper plates, ejector pins, etc.
An injection mold for making preforms (and other molded articles) typically includes one or more molding cavities for receiving molten resin and forming the preforms. To increase the flexibility of the molding assembly, interchangeable inserts can be inserted into bores in a cavity plate. FIG. 1 shows a portion of a prior art injection molding machine 10. One or more mold cavities 12 are usually defined between complementary shaped, generally cylindrical cavity inserts 14 and core inserts 15 that are arranged within bores (not separately numbered) defined in a cavity plate 16. The mold cavities 12 are aligned generally parallel to the direction of mold-clamping action (i.e., the mold-clamping axis).
For molded articles that have threaded neck portions, a split neck ring (not shown) cooperates with the core insert 15 to create the neck. A taper is typically provided at an end of the cavity insert 14 (also not shown) to help align the neck ring.
A hot runner assembly 18 communicates a flow of molten resin to melt channels 19 in one or more nozzle assemblies 20. A gate insert 22 is seated within the mold cavity inserts 14. A first profiled surface 24 on the gate insert 22 defines a receptacle to house the tip of the nozzle assembly 20. A second profiled surface 26 on the gate insert 22 defines a portion of the mold cavity 12. A gate 28 is provided in the gate insert 22 which provides fluid communication between each of the nozzle assemblies 20 and each of the mold cavities 12. Gate 28 is open or closed by a valve pin 29. Other types of gating, such as slide valves or thermal gating can also be used.
The molten resin that is injected into the cavities must be cooled to solidify the resin so that the molded preform can be removed from the mold cavity 12. It is desirable to cool the preform as quickly as possible so the preforms can be removed and a next injection cycle initiated with minimal time delay. To this effect, cooling channels 30 are typically provided in the cavity inserts 14 and gate inserts 22. A cooling fluid, such as water, is circulated through the cooling channels 30.
U.S. Pat. No. 5,472,331 teaches a core pin 14, which serves as a force transducer to detect pressure in the mold cavity. This is achieved by means of a strain gauge bridge 50. The strain gauge measures the elastic deformation of the pin resulting from the compensating force placed on the core pin during pressurization of the molding cavity.
US patent publication 2005/0236725 teaches a core 153 with an annular channel 160 in which an annular shaped piezoceramic insert 161 is positioned. The sensor can be used to sense pressure within the mold.
U.S. Pat. No. 5,972,256 teaches a core pin 10 with a core tip 22. The core pin 10 further includes strain gauges for measuring in mold pressure. When the molten material enters the cavity, the molten material pushes axially on the core pin 22. The strain gauges respond linearly to the applied force of the plastic entering the cavity. The information sensed can be used to accurately control the injection process.
U.S. patent application 2008/0085334 teaches a plug placeable in a hot runner manifold, the plug having sensing element. Responsive to the pressure sensed by the plug, the temperature of the heaters or pressure of the melt can be adjusted (see paragraph 0035).
U.S. Pat. No. 7,258,536 teaches various sensors, including a pressure sensor, and responsive to the reading from the sensors, controlling processing parameters, including heaters.
U.S. Pat. No. 5,176,859 teaches pressure transducers to measure in-mold cavity pressure and responsive to the measurement to control the clamp force.
U.S. Pat. No. 3,807,914 teaches sensing means mounted in the mold to measure molding cavity pressure—as the resin flows in the molding cavity, the resin engages the end of the ejector pin and applies pressure thereagainst. This pressure reflects the pressure of the resin present in the molding cavity. The ejector pin moves in response to the pressure and the movement is sensed to obtain the reading of the pressure of the resin in the molding cavity. Responsive to this reading, control algorithm controls heating means to maintain the pressure of the resin in the mold cavity at a pre-determined level.
U.S. patent application 2004/0142057 teaches using sensors to measure pressure in the cavity and a control unit that control a piezoelectric actuator to control/adjust cavity pressure.